New Study Of Tree Roots Will Alter Carbon-sequestration Models

"Scientists have been assuming," said principal investigator Roser Matamala, "that all the forests in the world behave the same way, whether they are evergreens or deciduous, and this is proof that they have differences." Matamala is an ecologist in Argonne's Environmental Research Division.

This data is important for modeling ecosystems. "We suggest," Matamala said, "that past studies using a uniform one-year turnover time for fine roots could have overestimated the amount of carbon dioxide the forests of the world can sequester each year."

The new data will change some carbon sequestration research.

Researchers are investigating carbon storage in soil and roots -- carbon sequestration -- as a possible way to recapture some of the excess atmospheric carbon that accompanies increasing industrial growth. This excess carbon is believed to be related to global warming.

Plants and trees process carbon naturally through photosynthesis, using atmospheric carbon dioxide to grow. When plant tissues eventually disintegrate, some of the carbon is sequestered in the soil as organic matter.

"Some forests would do a better job than others in taking up carbon dioxide from the atmosphere and placing it in the soil," Matamala said. "Pine trees have slow root replacement, which decreases the potential to accumulate carbon in the soil. In contrast, the fast root production in the sweetgum forest led to a rapid and significant increase in accumulation of soil organic matter."

Matamala's research was published in the Nov. 21, 2003, issue of Science and was featured as a "hot paper" by Thomson ISI on the ESI web site in November 2004. To date, the Science publication had already been cited in 20 peer-reviewed research publications.

"I think the paper has been cited often," Matamala said, "because we show that the one-year turnover rate for fine roots used in models would result in an overestimation the carbon sequestration potential of forests under climate change. We also describe a more reliable approach to studying root dynamics."

When building models of future carbon sequestration, researchers have used one year as the lifetime for fine tree roots to grow and decay into organic matter. This estimate was determined mostly by using mini-rhizotrons -- clear plastic tubes about 2 inches in diameter with "windows" about a half-inch square -- placed in the ground. Every two weeks researchers used a camera to look into the windows and review root growth and death. "With that method you might not see the long-lived roots through the tiny windows," Matamala said.

So she and her colleague Miquel Gonzalez-Meler of the University of Illinois at Chicago came up with the idea of using carbon-13 as a label to trace root turnover.

The research was performed at two mature tree plantations in the same growth belt in the southeastern United States. These Free-Air Carbon Dioxide Enrichment (FACE) sites are funded by the Department of Energy's Terrestrial Carbon Processes program, under the Office of Science's Office of Biological and Environmental Research.

A prototype carbon fumigation system was installed and tested at a FACE site near Duke University in Durham, N.C. Carbon dioxide is pumped through large pipes through treetop-high polyvinyl chloride pipes mounted on towers. A smart controller keeps the carbon dioxide level constant by making adjustments for wind speed and wind direction in the ring of trees being studied.

Researchers pump out carbon dioxide to achieve the higher levels scientists predict will be in Earth's atmosphere in 2050. Tree production increases with higher carbon dioxide levels.

Once the prototype was proven, more carbon dioxide delivery rings were built in the Durham loblolly pine forest. Three acted as controls, and three fumigated the trees with the expected 2050 level of carbon dioxide. The added carbon dioxide contained less carbon-13 than atmospheric air and was used as a tracer in the plant tissues and soil.

Three hundred miles west, a similar setup was built on a FACE site of sweetgum trees operated by Oak Ridge National Laboratory.

"The trees were exposed to the carbon dioxide with the carbon-13 tracer for five years," Matamala said.

Ecologists collected cores of soil near the trees, cleaned the material, and separated the roots. Using the carbon-13 label and mass spectrometry, researchers determined that the fine roots of sweetgum trees have shorter lives than fine roots of loblolly pines.

The mean lifetime of the very fine roots of the pines is 4.2 years, but they can last as long as 12 years, while sweetgum roots live 1.25 years but can last 4 years.

"The major implication for greenhouse management," said Gonzalez-Meler, "is that some forests won't transfer carbon from the atmosphere to soils at the speed we need to reduce global warming."

Researchers will continue to collect data from the sites. "We are interested in the different carbon sequestration dynamics of the forests," Matamala said. "The pines are more conservative than sweetgum. Sweetgums appear to have larger turnover of roots, investing more carbon in belowground structures than evergreens, like pines, do.

"In a future world you would try to breed trees with greater root production and turnover to help transfer atmospheric carbon into soils more effectively," Matamala said.

This research has opened new directions for investigation, including:

* Determining which carbon compounds turn over faster in roots,

* Investigating the discrepancies between conventional and isotopic methods, and

* Using the tracer to track carbon in other tissues in the tree and into the soil organic matter itself.

Argonne Environmental Research Division colleague Julie Jastrow was part of the research team. Her carbon sequestration work focuses on the dynamics of soil organic matter. Richard J. Norby of Oak Ridge National Laboratory and William H. Schlesinger of Duke University also contributed.